Method of making transparent electro-conductive surfaced plastic articles



Dec. 24, 1957 A. s. LOUIS 2,817,604

METHOD OF MAKING TRANSPARENT ELECTROCONDUCTIVE SURFACED PLASTIC ARTICLESFiled March 4, 1953 INVEN TOR. ARNOLD S. LOUlS *M H a AGENT UnitedStates Patent METHOD OF MAKING TRANSPARENT ELECTRO- CONDUCTIVE SURFACEDPLASTIC ARTICLES Arnold S. Louis, New York, N. Y., assignor to Myron A.Coler, Scarsdale, N. Y.

Application March 4, 1953, Serial No. 340,314

4 Claims. (Cl. 117-211) This invention relates to methods of makingelectrically non-conductive articles having an adherent electricallyconductive coating, and in particular such coatings which aretransparent.

In my copending applications entitled Improved Electrically ConductiveArticles and Method of Making Same, Serial No. 256,516, filed November15, 1951, now U. S. Patent No. 2,788,297, and Improved ElectricallyConductive Articles and Method of Producing Same, Serial No. 340,313,filed of even date, there is disclosed an electrically non-conductingarticle having an electroconductive surface.

There is disclosed herein a method of coating an electricallynon-conductive surface with conductive material by a simple processwhich may be used on even relatively soft readily scratchable surfacessuch as polished polymethylmethacrylate and other plastics and whichpermits the treating of large sheets.

The term plastics as used herein is intended in its chemical sense andnot in its physical sense.

The term plastics embraces any one of a large and varied group ofmaterials commonly referred to as plastics and resins, such aspolymethylmethacrylate, polyvinylchloride, polyethylene, polystyrene,polyvinylidenechloride and their copolymers.

In many applications for electrical insulators such as plastics it isdesirable that an electrically conductive surface be provided in orderto eliminate electrostatic charges. In electrical measuring apparatuselectrostatic charges cause disturbances to sensitive meters.Accordingly, a conductive surface is sought for the meter casingobservation window. In another typical case, that of aircraft, theelectrostatic charging of the plastic cockpit canopy and electricallynon-conductive plastic surfaces covering radio and radar antennas createelectrical interference with radio communication and radar signals.

An electrically conductive coating applied to an electricallynon-conductive surface can be used as a resistive heater elementtomaintain the electrically non-conductive surface or its surroundings ata desired temperature.

The prior art shows various partially successful methods of applyingcarbon and other conductive films. Thus, surfaces possessing sufiicientinherent adherence such, for instance, as the surface of a wax object,will readily attach to itself a relatively continuous graphite layerwhich can be electroplated. Such films are rather delicate and usuallycompletely opaque.

Again, it is conventional to apply graphite in dispersion in filmforming adhesives to the surfaces of insulator objects. It has not beenpossible to obtain such coatings which are sufiiciently conductive, andat the same time reasonably transparent. Furthermore, there are problemsconnected-with peeling and loose adherence of the coating and withcrazing of insulator plastics to which they are applied.

A conventional device to obtain transparent conductive coatings is theapplication of organic films containing water soluble electrolytes. Suchfilms are generally very unstable as to their electrical properties andcan be easily washed from the insulator base. This invention, bycontrast, is concerned with obtaining a coating which is willcientlythin to be transparent and yet sufficiently adherent to withstandconsiderable abrasion and Washing.

Accordingly, it is an object of this invention to provide an improvedmethod for applying an electrically conductive coating to anon-conductive plastic article.

Another object of this invention is to provide a method of obtaining auniform electro-conductive surface on large electrically non-conductivesurfaces.

A particular object of this invention is a method of applying atransparent electrically conductive surface to electricallynon-conductive articles.

Other objects and advantages of this invention will appear more fullyand clearly from the following description of illustrative embodimentsthereof taken in connection with the appended drawings in which:

Figure 1 shows an apparatus for coating articles.

Figure 2 shows in section an enlarged view of a typical applicator.

Figure 3 shows a rotary apparatus used for coating articles.

.Briefiy stated, the process of this invention comprises coatingspherical bodies with finely divided electrically conductive materialand then rolling the coated spheroids over a surface to be coated sothat the electrically conductive material is transferred thereto.

In one method of this invention a tray 2 is pivoted on a fulcrum so thatit may be rocked see-saw fashion. An electrically non-conductive articlesuch as a sheet of polymethylmethacrylate 6 is placed on the bottom ofthe tray 2 and at least partially covered by a layer of applicatorscomprising coated spheres 8. The tray is then oscillated about fulcrum 4causing the applicators to roll from one end of the tray to the other.After a period of time a uniform coating is formed on the surface.

A typical applicator is shown in greater detail in Figure 2 whereinsphere 22 is coated with finely divided parti cles 24 of electricallyconductive material.

In general, it is preferred that the applicator particles be roundrather than angular in shape to avoid scratching of the insulatorsurface and to permit uniform rolling. Larger and heavier applicatorsare preferred, the harder the surface to be coated and the higher theconductivity which it is desired to impart to the treated surface.

Thus, polystyrene beads about 1 mm. in diameter have been foundeffective in coating polymethylmethacrylate sheet. Steel balls, /s inchin diameter, have been effective in applying rather heavy conductivecoatings to polymethylmethacrylate. I

In general, applicators should not be smaller than 0.0025 inch norlarger than 0.25 inch. Smaller applicators lack the necessary mass to beeffective. With larger applicators it is inconvenient to obtain themultitudinous contacts which are needed to secure the full advantage ofthis invention.

Applicators may be made of any material which is stable physically andchemically under the conditions of use. They should not have such anadhesive surface as to bond the conductive material inextricable tothemselves. Suitable applicator materials include glass, polystyrene,steel balls and ceramics.

The electrically conductive coating material must be finely divided andchemically stable under conditions of impact. The range of fineness iscritical and the particles should not be coarser than ten microns andpreferably finer than two microns in the smallest dimension. Particlesof laminar habit are particularly convenient for the practice of thisinvention.

Applicator particles may be coated with conductive material by any ofseveral techniques, as for instance, by exposure to the sooty productsof the incomplete combustion of carbonaceous gaseous fuels or by wettingwith a suspension of conductive material in a liquid followed by dryingof the coated particles. The preferred method involves tumbling theapplicators with finely divided conductive material until a uniformcoating is obtained.

Only a small amount of conductive material, usually 0.01 to 0.5% byweight of the quantity of applicator used is needed. After applicatorshave been used for some time it will be necessary to replenish thecoating of conductive material.

Figure 3 discloses a typical tumbling barrel 32 in which is jigged asheet of thermoplastic 34 which has been heated and bent to conform withthe interior configuration of barrel 32. A limited number of coatedimpactors 36 are introduced into the barrel and the barrel slowlyrotated as by rotation of rollers 38 so that the applicators slowly rollover the surface of the plastic Without cascading as occurs in thenormal operation of a tumbling barrel as in burnishing operations. Aftertreating, the plastic sheet may be reheated and flattened to itsoriginal shape.

The invention has been described with respect to flat sheeting howeverit is applicable to treating of any complex shape over or in which theapplicators may be rolled.

Plastics which may be treated by the method of this invention includepolymethylmethacrylate, polystyrene, phenolformaldehyde,polyvinylchloride and polyviny1i-- dene chloride, it being understoodthat this listing is merely representative and not to be construed aslimiting.

The following examples illustrate the invention:

Example 1 3000 grams of polystyrene beads and 3 grams of graphite (grade200-10 made by Dixon Crucible Company) were charged to a 10 gallon drumand the whole rotated at 40 revolutions per minute for 10 hours. At theend of this time a sheet of polymethylmethacrylate was mounted so as tooccupy a section of the circumference of the drum, all of the charge inthe drum being placed on one side of the sheet. Rotation was continuedfor 10 hours at 20 R. P. M., the speed of rotation being such that onlya rolling motion of the charge took place. The sheet had a relativelyuniform coating having a resistance of 0.6 megohms per square and alight transmission of 70%.

Example 2 grams of graphite and 1000 grams of XXX grade polystyrenebeads made by Kopper Co., Inc. were charged to a 2000 cc. bottle whichwas rotated at about 40 revolutions per minute for 13 hours.

A porcelain lined metal tray 16 inches long and inches wide was arrangedto pivot about the mid point of its length while one end was moved upand down through a stroke of 2 inches at a rate of about 50 cycles perminute. A sheet of polymethylmethacrylate thick and 4 inches square wastaped to the bottom of the pan. The polystyrene beads, coated asdescribed above, were placed on top of the sheet. The pan was startedoscillating.

The beads moved back and forth on the sheet with a rolling motion. Theoscillation was continued for 2 /2 hours after Which the sheet wasremoved, washed with soap and water, dried and examined. The sheet borea uniform conductive coating having a resistance of 4 megohms per squareand light transmission of 84%. The sheet was free of scratches.

This Example 2 represents my best present judgment as to the preferredembodiment of my invention.

Example 3 2 grams of graphite of the 200-10 grade and 2000 grams of /3"steel balls were charged into a 500 cc. bottle which was then tumbledfor 5 hours at 40 revolutions per minute so as to form applicators. Theapplicators were then charged into a 16" x 10" x 1" tray having apolystyrene sheet on the bottom. The tray was tilted about 5000 timescausing the applicators to roll across the polystyrene each time.

After washing with soap and water the polystyrene was found to have alight transmission of and a resistance of 100,000 ohms per square on thecoated face. The surface was not pitted or scratched as a result of thetreatment.

Although it has been pointed out that adhesives are not required incarrying out this invention, it is to be noted that pretreatment of thesurface before coating may be helpful under some circumstances. Suchtreatment may include the cleaning of the surface or the heating of thesurface until it is tacky.

There is shown in Figure 1 infra-red heat lamps 10 which may be used toheat the surface of the plastic.

In carrying out the invention the atmosphere within the rotating barrelmay be adjusted to meet the requirements of the particular combinationof the coating material and article to be coated such as low or highambient temperature and/ or a reducing gas atmosphere to preventoxidation of the various surfaces.

In the referenced copending application entitled Improved ElectricallyConductive Articles and Method of Making Same filed of even date, it wasdisclosed that the coated plastic articles may be advantageously treatedwith a solvent, for the plastic, to fix the coating more permanently.This treatment may be used on the product of this process.

Although preferred embodiments have been disclosed, it will beunderstood that modifications may be made within the spirit and scope ofthe invention.

I claim:

1. The process of coating a transparent substrate selected from thegroup consisting of synthetic resins and cellulose acetate with anelectrically conductive adherent coating of graphite, consisting of thesteps of coating a plurality of substantially spherically shapedparticles with finely divided graphite and then gently rolling saidplurality of coated spherical particles over said transparent substratesolely by their own weight so as to avoid impacting said coatedparticles against said substrate, and maintaining said rolling stepuntil the substrate has deposited thereon a transparent electricallyconductive coating.

2. The process of claim 1 wherein said substrate is polystyrene.

3. The process of claim 1 wherein said substrate is polymethylmethacrylate.

4. The process of claim 1 wherein said substrate is cellulose acetate.

References Cited in the file of this patent UNITED STATES PATENTS706,701 Thurston Aug. 12, 1902 1,385,184 Meade et al. July 19, 19212,292,026 Gillet Aug. 4, 1942 2,554,723 Webb May 29, 1951 OTHERREFERENCES Printed Circuit TechniquesNational Bureau of StandardsCircular 468 (only pages 5 to 9 relied upon).

1. THE PROCESS OF COATING A TRANSPARENT SUBSTRATE SELECTED FROM THEGROUP CONSISTING OF SYNTHETIC RESINS AND CELLULOSE ACETATE WITH ANELECTRICALLY CONDUCTIVE ADHERENT COATING OF GRAPHITE, CONSISTING OF THESTEPS OF COATING A PLURALITY OF SUBSTANTIALLY SPHERICALLY SHAPEDPARTICLES WITH FINELY DIVIDED GRAPHITE AND THEN GENTLY ROLLING SAIDPLURALITY OF COATING SPHERICAL PARTICLES OVER SAID TRANSPARENT SUBSTRATESOLEY BY THEIR OWN WEIGHT SO AS TO AVOID IMPACING SAID COATED PARTICLESAGAINST SAID SUBSTRATE, AND MAINTAINING SAID ROLLING STEP UNTIL THESUBSTRATE HAS DEPOSITED THEREON A TRANSPARENT ELECTRICALLY CONDUCTIVECOATING.